1. Field of the Invention
The present invention relates generally to an information recording medium with respect to which information can be optically recorded, erased, rewritten, and reproduced, and to a method of manufacturing the same.
2. Related Background Art
In a phase-change information recording medium, information is recorded, erased, and rewritten using a recording layer that is transformed in phase reversibly between a crystal phase and an amorphous phase. When this recording layer is irradiated with a high power laser beam and then is cooled rapidly, a portion thus irradiated is changed to be in an amorphous state and as a result, a recording mark is formed. Similarly, when an amorphous portion of the recording layer is irradiated with a low power laser beam and then is cooled slowly, the portion thus irradiated is changed to be in a crystal phase and as a result, a recording mark is erased. Therefore, in the phase-change information recording medium, the recording layer is irradiated with laser beams having powers modulated between a high power level and a low power level, so that new information can be rewritten while previous information is erased.
When information is to be rewritten, atoms move within the recording layer as the recording layer is transformed in phase between the crystal phase and the amorphous phase. As a result, in a conventional information recording medium, when rewriting is repeated, atoms may be concentrated locally to vary the thickness of the recording layer and this may cause deterioration in signal quality in some cases. Such repeated-rewriting performance is deteriorated particularly with the increase in recording density. The reason is that when the recording density increases, the intervals between adjacent recording marks are shortened and therefore the influence of the concentration of atoms in the adjacent recording marks increases.
In order to prevent the repeated-rewriting performance from being deteriorated, it is necessary to reduce the thickness of the recording layer to suppress the atom movement. In addition, the reduction in thickness of the recording layer also is a technique required to obtain a high density information recording medium with two recording layers. However, the reduction in thickness of the recording layer makes it difficult for atoms to move. Therefore, the crystallization rate of the recording layer decreases. The decrease in crystallization rate results in the deterioration in signal quality in a high density information recording medium in which small recording marks must be recorded in a short time. In addition, when the crystallization rate decreases, deterioration in crystallization sensitivity with time and that in erasing rate with time tend to occur. In other words, with the increase in recording density, it becomes difficult to achieve both the improvement in the repeated-rewriting performance and the suppression of the deterioration in crystallization sensitivity with time.
In order to improve the repeated-rewriting performance, a recording layer containing Te, Ge, Sn, and Sb has been reported (see JP 2(1990)-147289 A).
In the above-mentioned conventional recording layer, however, the crystallization rate was high but the repeated-rewriting performance and long-term reliability of the crystallization sensitivity in high density recording were not sufficiently high.